Implement "per-epoch processing - validator registry" section

[hwwhww]

What is wrong?

Spec: per-epoch processing - validator registry https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#validator-registry

We don't have this section yet.

It could be 4 parts:

1. Common logic: "update previous_epoch_calculation_slot and previous_epoch_start_shard.... " part
2. Conditions: "If the following are satisfied... " part
3. "If a validator registry update does not happen"
4. "If a validator registry update does happen"

For MVP, at least we need the part 1-3.

How can it be fixed

• In eth2/beacon/state_machines/forks/serenity/epoch_processing.py, add process_validator_registry(state: BeaconState, block: BaseBeaconBlock, config: BeaconConfig) -> BeaconState.
• Call process_validator_registry in eth2.beacon.state_machines.forks.serenity.state_transitions. SerenityStateTransition.per_epoch_transition:

    def per_epoch_transition(self, state: BeaconState, block: BaseBeaconBlock) -> BeaconState:
        # TODO: state = process_receipt_roots(state, block, self.config)
        # TODO: state = process_justification(state, block, self.config)
        # TODO: state = process_rosslinks(state, block, self.config)
        # TODO: state = process_rewards_and_penalties(state, block, self.config)
        # TODO: state = process_ejections(state, block, self.config)
        state = process_validator_registry(state, block, self.config)
        # TODO: state = process_final_updates(state, block, self.config)

        return state

Note: recent incoming update: ethereum/consensus-specs#476 will affect it.

[hwwhww]

including part 1~3 into #225

[hwwhww]

working on update_validator_registry